Cholesteric liquid crystals (Ch’s) are formed by organic molecules that pack in space into periodic helical structure and are capable to selectively reflect light in the visible part of the spectrum when the period of the helix is in the submicron range. This property is attractive for designing optical elements such as band-pass filters, mirrors, low-threshold lasers, etc. However, the Ch period is hard to tune continuously by electric or magnetic fields. In chiral mixtures of flexible dimers, an applied field can create an oblique helicoidal structure (ChOH) which is stable in a wide range of applied field maintaining its axis parallel to the field (Xiang, J. et.al, Phys. Rev. Lett. 112, 217801 (2014); Xiang, J. et.al, Adv. Mater. 27, 3014 (2015)). Both the ChOH period and the conical angle depend strongly on the field, which enables electrically tunable Bragg reflection in a broad spectral range from ultraviolet to infrared. We explore the Bragg diffraction at the ChOH periodic structure as a function of the electric field, surface anchoring, angle of light incidence, and polarization. In ChOH mixtures doped with azobenzene-based photosensitive compound, we show that the ChOH period can be tuned by UV irradiation because of trans-to-cis isomerization. Finally, Bragg reflection at the ChOH periodic structure can be used to measure elastic constants of Ch phase. The work is supported by NSF grant ECCS-1906104.

The use of solid electrolytes based on zirconia stabilizing scandium oxide results in a material with high ion conductivity. Electrolytic membranes made of such a material can significantly lower the operating temperature of an electrochemical device while maintaining high conductivity, which is very important for increasing the service life and reliability of electrochemical reactors, solid oxide fuel cells, electrolyzers and sensors.

The materials based on zirconia stabilized with scandium oxide phase boundaries are defined only approximate. This is due to the existence of metastable phases in this system and the dependence of the phase composition on the method and conditions of material synthesis. Additional doping of ZrO₂ ‑ Sc₂O₃ solid solutions with rare-earth oxides makes it possible to obtain stable cubic solid solutions with high conductivity.

The aim of this work is to assess the effect of the introduction of dopant ytterbium oxide into ZrO₂ – 9 mol.% Sc₂O₃ solid solutions in an amount from 0.5 to 2 mol.% on the phase composition, structure, and electrophysical properties of the material.

Single crystals of solid solutions were grown by directional crystallization of the melt in a cold container. The phase composition of the samples was controlled by Raman spectroscopy and X-ray diffractometry. The crystal structure was investigated by transmission electron microscopy. The study of the transport characteristics of crystals was carried out by the method of impedance spectroscopy in the temperature range 450 ‑ 900 C in the frequency range 1 Hz – 5 MHz.

It is shown that the stabilization of ZrO₂ together with 9 mol.% Sc₂O₃ and 1 mol.% Yb₂O₃ makes it possible to obtain transparent homogeneous crystals with a pseudocubic structure, which have high phase stability. The conductivity of the crystals depending on the concentration of Yb₂O₃ is nonmonotonic. The (ZrO₂)_0.9(Sc₂O₃)_0.09(Yb₂O₃)_0.01 crystals have a maximum conductivity in the investigated temperature range. An increase in the amount of Yb2O3 in the composition of the solid electrolyte to 2 mol% led to a decrease in the conductivity of the crystals due to the formation of oxygen-ion clusters. An increase in the concentration of ytterbium oxide to 2 mol% led to a decrease in the conductivity of the crystals due to the formation of oxygen-ion clusters.

The work was carried out under financial support of the Russian Science Foundation (grants № 19-72-10113)

Materials based on partially stabilized zirconia are widely used as structural non-metallic high-strength and wear-resistant materials, thermal barrier and protective coatings, as well as bioinert materials for medicine. The tetragonal phase of solid solutions based on zirconia is stabilized by doping with yttrium, cerium oxides, or oxides of other rare earth elements.

One of the possible ways to optimize the mechanical characteristics of ZrO₂-Y₂O₃ solid solutions crystals is to partially replace Y₂O₃ with oxides of other rare-earth elements. This work presents the results of studying the phase composition, structure, and mechanical properties of ZrO₂-based crystals stabilized with yttrium oxide and co-doped with neodymium, erbium, or ytterbium oxides with a total concentration of 3.2 mol.%.

The crystals were grown by directional melt crystallization in a cold container. The phase composition of the crystals was determined by X-ray diffractometry and Raman spectroscopy. The crystal structure was investigated by transmission electron microscopy. The microhardness and crack resistance of crystals were measured by the indentation method.

The study of the phase composition and structure of the crystals showed that, while maintaining the total concentration of codoning oxides, a change in the degree of substitution of Y³⁺ cations for dopant cations affects the quantitative ratio of phases, the degree of their tetragonality, and the size of twins.

The study of the mechanical characteristics of crystals, such as fracture toughness and microhardness, showed that co-doping has an insignificant effect on the change in microhardness values. The value of the crack resistance of crystals increases with an increase in the radius of the rare earth element of the co-doped oxide. The study of the monoclinic phase distribution in the region of the indenter indentation is carried out. Comparison of the obtained data on the dependence of the tetragonal - monoclinic phase transformations intensity with the data on the fracture toughness of crystals shows a general tendency towards a decrease in the values of fracture toughness with a decrease in the intensity of the tetragonal - monoclinic phase transformations.

The work was supported by research grants № 18-13-00397 of the Russian Science Foundation.

Over the last two decades, we have witnessed a paradigm change in the way we characterize materials using electron microscopy. This latest revolution in resolution began in the late 1990’s with the first successful implementation of an objective lens aberration corrector, which improved the spatial resolution of transmission (TEM) and scanning transmission electron microscopy (STEM) by more than a factor of two to below 50 pm. These developments were followed by faster, more sensitive direct electron (CMOS) detectors, monochromated electron sources for electron spectroscopy and, most recently, magnetic field-free lenses. As the result of these transformational discoveries, we are now able to study materials with unprecedented resolution, sensitivity and precision. While spatial and energy resolutions better than 60 pm and 10 meV have been reported, aberration-corrected TEM has also enabled a large variety of in-situ experiments at close to atomic resolution.

In my talk, I will highlight several examples where atomic-resolution in-situ, multi-modal characterization and first-principles modeling are used to unravel the fundamental structure-property relationships of materials with potential applications in high-efficiency photovoltaic devices, rechargeable batteries or electro-catalysis. I will further introduce a novel approach to characterizing phase transitions in fluids at high spatial and isotopic resolution. I will conclude by presenting my vision for the future of electron microscopy, including new instrument designs as well as operando multi-modal methods combining electron scattering with transport measurements.

Topological defects (TDs) constitute topologically protected frustrated regions in a host field of an ordered manifold. They are ubiquitous in nature and appear at all scales, including the realms of particle physics, condensed matter, cosmology… The sole condition for their existence is symmetry-breaking. For example, the first theory of phase transition quench-driven coarsening of TDs was developed in cosmology to model events in the early inflationary universe. Furthermore, it might be that localized TDs represent fundamental particles of the Standard Model if relevant physical fields constitute fundamental natural entities. Due to their topological origin, TDs exhibit several universalities. Therefore, it is of interest to identify systems where TDs are easily experimentally accessible, enabling detailed and well-controlled analysis of their universal behavior, cross-fertilizing knowledge in different areas of physics. In this respect liquid crystals (LCs) represent an ideal experiment testbed to study TDs. LCs display numerous phases and structures reached via continuous symmetry breaking phase transition, which can host a rich diversity of TD structures. Furthermore, LCs possess a unique combination of liquid character, orientational and/or translational order, softness (capability to respond strongly to even weak stimuli¹), and optical anisotropy and transparency. This symbiosis of properties allows relatively ease of TDs creation, their simple observation (e.g., using polarizing microscopy), stabilization and manipulation of their configurations. Note that, in general, it is difficult to stabilize TDs because they are energetically costly. In this presentation I will show our investigations of TDs in the nematic uniaxial phase, representing the simplest LC configuration, displaying only orientational order. Despite its simplicity, the nematic phase exhibits a rich diversity of defect configurations. We demonstrate that a simple plane parallel cell that confines a nematic LC could host diverse complex and multistable configurations of TDs, which we stabilized using the AFM scribing method². These competitive states could be reversibly and robustly reconfigured by appropriate external electric fields. Furthermore, we show that complex lattices of line defects, which are otherwise unstable or stable in a narrow interval of temperatures, could be stabilized efficiently by doping LCs with appropriate nanoparticles, owing to the universal defect core displacement mechanism³. We demonstrate that such TD configurations have potential for diverse applications, particularly in nano- and biotechnology: e.g., for nanotechnology-based devices based on reconfigurable conducting nanowires, tunable photonic devices, sensitive sensors… Furthermore, our study of TDs might provide some insight into still unresolved problems of fundamental physics. Namely, LCs could exhibit so-called “chargeless” twist disclinations, which commonly decay into a defectless state. Twist TDs could simultaneously act as defects and antidefects³, and such neighboring pairs could be mutually annihilated. These configurations bear some resemblance to intriguing Majorana particles that, furthermore, could play the role of neutrinos, the physics of which is still unresolved. We show that isolated twist loops could be stabilized by toroidal topology. The latter possesses regions exhibiting positive and negative Gaussian curvature. 2D studies^4,5 reveal that such regions attract TDs exhibiting positive and negative topological charges, respectively. Our preliminary investigations reveal that similar mechanisms also could be applied in 3D.

The electronic and thermoelectric properties of half-heusler compound NiTiSi has been studied using density functional theory and Boltzmann transport theory within the constant relaxation time approximation. NiTiSi is found to be an indirect bandgap semiconductor with a band gap of 0.747 eV. Seebeck coefficient greater than 200μV/K at 1000 K is observed. The calculations suggests that p-type doping can significantly improve the thermoelectric properties of the compound with a maximum value of 0.13 at 1000 K.

The electronic, magnetic and optical properties of double perovskites Pb₂XOsO6 (X=Co, Ni) have been studied by using density functional theory within generalized gradient approximation. By replacing Co from Pb₂CoOsO₆ (PCOO), Pb₂NiOsO₆ (PNOO) is formed. With electron doping, we observed electronic and magnetic phase transition as PCOO and PNOO are found to be A-type anti-ferromagnetic and ferrimagnetic materials having metallic and half metallic features respectively. We noticed significant changed in band structure and density of state as we apply coulomb correlation (U) as well as spin orbit coupling. We calculated the real and imaginary part of dielectric function, optical conductivity and loss function and finally analyzed inter-band transition to the optical properties with band structure.

The knowledge of the structure-property relationships is basic for the comprehension of the action mechanism of all materials. In particular, many solid-state compounds are available in the form of microcrystalline powder. In this case, the discovery of the crystal structure by diffraction methods is usually not easily achieved even in the case of a small molecule. Difficulties are met to attain a correct interpretation of the experimental X-ray powder diffraction pattern, due to peak overlap, incorrect background estimation, and possible preferred orientation effects. Modern instrumentation, theory and software are addressed to overcome such limits for accomplishing a successful crystal structure solution [1, 2]. Powder solution methods can be classified into two main groups: Reciprocal space methods, as Direct Methods (DM); Direct space methods, as Simulated Annealing (SA). They present different limits and advantages. The choice of the best strategy to adopt depends on some factors: experimental data quality, experimental resolution, peak overlap, structure complexity expressed in terms of number of non-hydrogen atoms in the asymmetric unit and/or number of degrees of freedom of the structure model, as well as available information on the expected molecular model.

EXPO [3] is an advanced crystallographic software capable to perform the solution by DM or SA, and also to efficiently combine them [4].

The conditions requested for a profitable outcome by DM and SA, respectively, are discussed and compared by using examples. Useful suggestions, for preferring one method rather than the other one, are given and the opportunity of using both in EXPO is investigated.

References

[1] Powder diffraction Theory and Practice; R.E. Dinnebier, S.J.L. Billinge Eds., The Royal Society of Chemistry, Cambridge, 2008.

[2] International Tables for Crystallography, Volume H, Powder Diffraction; C.J. Gilmore, J.A. Kaduk, H. Schenk Eds., Wiley: New York, 2019.

[3] A. Altomare, C. Cuocci, C. Giacovazzo, A. Moliterni, R. Rizzi, N. Corriero, A. Falcicchio. J. Appl. Cryst. 2013, 46, 1231.

[4] A. Altomare, C. Cuocci, N. Corriero, A. Falcicchio, R. Rizzi, Crystals 2019, 10(1), 16.

The search for mononuclear lanthanoid-based single-ion magnets (SIMs) has increased the interest in some coordination environments with low coordination numbers, in combination with uniaxial symmetry, as they could maximize the anisotropy of complexes of oblate lanthanoid ions, as disprosium(III). ions as disprosium(III). In this sense, the pentagonal-bipyramid geometry can have ground-state doublets with perfect axiality, and therefore such complexes can be good candidates for SIM. In our particular case, we have used a well-known open planar pentadentate chelating Schiff base ligand as 2,6-bis(1-salicyloylhydrazonoethyl)pyridine) (H₄daps) for the synthesis of air-stable pentagonal-bipyramidal Ln^III complexes (being Ln: Dy and Er, oblate and prolate, respectively), in order to compare their structures. Thus, reaction of H₄daps with (CH₃)₄N·5H₂O, and the corresponding LnCl₃·hexahydrate has yielded heptacoordinate [(CH₃)₄N][Ln^III(H₂daps)Cl₂] complexes, where the tetramethylammonium cation is acting as counterion of pentagonal-bipyramidal Ln^III complexes, which are bearing two chloride atoms in apical positions. As both complexes crystallized as single crystals, we can compare their crystal structures as well as with other related complexes in literature, but containing different counterions, trying to see their influence on other properties of the compounds, as their magnetic behavior.

Cyclometallated compounds have been extensively studied, in particular those with palladium and platinum. This is because of their possible applications in medicinal chemistry, as anticancer or antimicrobial agents; in some cases with similar results as cisplatin, carboplatin or oxaliplatin. Also remarkable is their use as homogeneous catalysts, for example, in cross coupling reactions such as Suzuki-Miyaura or Mizoroki-Heck.

Herein we report the synthesis of different thiosemicarbazone ligands, which will be reacted with a palladium or platinum salt, to give the corresponding cyclometallated compounds; their reactivity with bis(diphenylphosphino)methane (dppm) will be studied.

Characterization has been carried by elemental analysis, IR spectroscopy, ¹H and ³¹P NMR spectroscopy. Also, 1c has been studied by X-ray diffraction.